Threaded fastener technology is basic to the construction or fabrication of most articles of manufacture, such as machines, automobiles, trains, planes, engines, and the like. Threaded fasteners may be bolts, screws, studs, rods, or other substantially round members having uniform, non-uniform or tapered external helical threads that are screwably engaged in mating threaded fasteners, such as nuts or holes having substantially matching internal helical threads. For proper engagement of the externally and internally threaded fasteners, the longitudinal axis of the externally threaded member typically must be substantially collinear with the longitudinal axis of the internally threaded member. Further, proper engagement of externally and internally threaded fasteners typically requires that the peak of the external thread of the male helix is aligned with the root of the internal thread of the mating female helix. While this disclosure relates to any thread form, the International Organization for Standardization (ISO) metric screw thread will be illustrated as an example of a standard thread, as shown in FIG. 1. As used in this application, the designation “thread” may apply to an entire fastener thread helix, or any partial section(s) of the helix, wherein a thread may comprise a partial winding or several windings around the shank of a fastener, in the case of a male fastener member, or a partial winding or several windings around an interior hole, in the case of a female fastener member.
Failure to achieve proper thread engagement between externally and internally threaded fasteners during assembly is typically caused by one of two threading conditions that occur as the opposing helices engage and parts are rotated relative to one another. The first, typically termed “cross threading,” occurs when there is both angular and linear axial misalignment between the externally thread member, typically a bolt or screw, and the internally threaded member, typically a nut or threaded hole. Specifically, cross threading is the result of the two members attempting to engage at least one-half pitch out of linear alignment while the respective helix axes are also angularly misaligned.
FIG. 2 illustrates cross threaded male and female fastener members. When fastener members are cross threaded, the male fastener member 10 member is not collinear with the female fastener member 30, and the threads wedge as the threaded helices are rotated against one another. If rotation continues when the threads are in such a wedged condition, then the threads of one or both of the members will typically be structurally damaged.
The second threading condition that causes failure to achieve proper thread engagement is normally termed “false threading,” which occurs when the two helices are one-half pitch out of alignment so that the threads engage crest-to-crest rather than crest-to-root.
FIGS. 3A and 3B illustrate a false threading condition, wherein the helix axes are not misaligned, but rather they are essentially collinear. Internal threads are often manufactured with a slight depression, groove or fissure 32 at the crest of the internal threads. False threading occurs when the crest of an external lead thread 17 engages the groove 32 at the crest of the internal threads, so that the two helices are one-half pitch out of alignment. FIG. 3B is a close-up view of the false threading condition of FIG. 3A and illustrates how such grooves 32 at the crests of internal threads, while usually small in the main body of the internal thread helix, can be somewhat deeper and wider in the entry end of the helix—the lead thread section of the internal thread. In such a case, it is possible for the peak of the lead thread 17 of the external thread helix (and/or the first full thread of the helix) to attempt to enter the normal helical root in the opposing internal helix, but to instead engage the internal thread helix at the groove 32 in its crest.
Several characteristics of lead threads of internal helices currently being widely manufactured greatly increase the likelihood of false thread failures.
First, FIGS. 4A-4F illustrate how typical internal lead threads 31 of a female fastener member 30 tend to have a depression, groove or fissure 32 at their crests, which tend to cause false thread failures. These grooves 32 can be both deep and wide at the entry end of the helix. As the groove 32 progresses away from its beginning into the internal helix, it tends to narrow and become shallower, and in most fasteners, largely disappears over approximately one-half revolution of the internal helix. As such, the surface that forms the deepest point in the groove 32 tends to grow away from the axis of the internal thread helix at a rapid rate as the groove 32 in the internal lead thread 31 closes and approaches the full thread.
Second, FIGS. 5A-5B illustrate two side views of a male fastener member 10 having fastening threads 15, wherein the lead thread 17 can have a profile that usually is pointed and/or has some sort of protruding feature at its peak, which tends to cause false thread failures. FIG. 6 shows a side view of a male fastener member 10 having fastening threads 15 and an anti-cross thread 16, wherein the lead thread 17 has a typical profile with a pointed peak. FIG. 7 shows a side view of a male fastener member 10 having fastening threads 15, an anti-cross thread 16, and a dog point 22, wherein the lead thread 17 has a typical profile including a combination of curved and flat surfaces with a pointed peak. When the pointed peaks of the lead threads of male fastener members engage the grooves in the crests of the internal threads illustrated in FIGS. 4A-4F, false thread failures can occur.
Third, the relative helix angles of the peaks at the crests of the internal and external lead threads can cause false thread failures to occur. The internal and external lead threads tend to have dissimilar helix angles because of their respective methods of manufacture. If the helix angle of the external thread peak of the lead thread is greater than the helix angle of the internal peak of the lead thread, then the external peak is curved more severely. This means that during initial assembly, when the two lead threads make contact, their peaks are not essentially parallel. The more severely curved thread, i.e., the one with the larger helix angle, will tend to approach or intersect the other thread at one point on the thread. At the intersection point on the thread, the pointed peak of the external lead thread can enter into the groove in the crest of the internal thread so as to cause a false thread condition. Other orientation circumstances where the axes of the two threads are not collinear can also lead to false thread failures.
When these characteristics exist individually or in combination to allow the external lead thread pointed peak to inadvertently enter into the groove in the crest of the internal lead thread, that external peak may act as though it had threaded into a normal internal thread. Because there is no path out of the groove, the point of the external lead thread may continue to follow the groove as the fastener members are rotated relative to each other. The groove, however, very quickly becomes narrower and closes, as shown in FIGS. 4A-4F, such that there is no path for the external thread pointed peak to continue to thread. The helix angle of the line formed at the bottom of the groove in the internal lead thread is typically somewhat greater than the helix angle of the line formed at the pointed peak of the crest of the external thread. Because these lines are not parallel, i.e., the helix angles are different, the threads quickly intersect and the pointed peak of the external lead thread quickly contacts a surface in the groove in the crest of the internal thread. Thus, the pointed lead thread of the external fastener member is only able to thread freely for a few degrees of rotation until it grows too big for the shrinking groove in the crest of the internal thread, giving it no path to continue threading. Such engagement often results in the pointed peak of the external thread becoming lodged or jammed in the groove in the crest of the internal thread. Continued relative rotation of the fastener members beyond such false thread jamming point often damages or shaves off a piece of either thread, leading to structural failure of both thread helices.
Further, many external lead threads manufactured currently have a shape such that they present a bump or sudden increase in height, particularly where the lead thread is very short. If the lead thread is less than one half pitch in length (less than 180° around the shank), it may increase in height rapidly and thus be more susceptible to false threading. When an external lead thread having a bump or sudden increase in height is threaded into a groove in the crest of an internal thread, false thread failures can occur, as described above.
Various types of lead threads are in use in industry today. In all threaded fastener arts, except tapping screws, the lead thread is utilized to feed or “lead” the external thread helix into space between adjacent windings of the internal thread helix. In practice, several lead thread shapes have been utilized for this purpose. The vastly more common of these, used on most standard threaded fasteners, is a lead thread device that historically has utilized part of the first turn of the helix to grow from a zero height to the full thread height and simultaneously to grow wider in breadth. During typical manufacturing processes, this lead thread is allowed to take whatever free-form shape allows it to be easily manufactured. This shape usually includes one 60 degree flank on the side of the helix closest to the fastener head and one free-form flank on the side farthest from the fastener head. The profile of the lead thread varies significantly with manufacturing process and from fastener design to fastener design. Due to the variations inherent in manufacturing methods utilized to thread roll this lead thread section, the shape of this section is usually inconsistent in its shape and unpredictable in its growth rate. Variation is most notable in shape and linear growth in the ridge formed at the peak of the external lead thread. (See FIGS. 5A and 5B). Typical lead threads typically have a relatively sharp point at the crest of the lead thread because only one flank has a flat surface at the standard 60° angle, while the opposite flank surface is free-form at an angle much greater than 60°, and the two flanks connect directly with each other, rather than through a flat surface parallel to the longitudinal axis of the fastener shank like the remaining standard threads.
One “non-standard” fastener includes an anti-cross thread more fully described in U.S. Pat. No. 5,730,566, incorporated in its entirety herein by reference. These non-standard fasteners include three threads: the lead thread 17, the anti-cross thread 16, and the fastening threads 15. (See FIG. 6). Typical lead threads on fasteners having anti-cross threads are somewhat different than lead threads on standard fasteners, but they are just as susceptible to variation in profile and length. The profile of a typical lead thread of a fastener having an anti-cross thread has three common characteristics in every section of the lead thread helix.
First, as shown in FIG. 7, the lead thread flank 17b closest to the anti-cross thread 16 tends to maintain a curve not unlike one half (or less) of that seen in the anti-cross thread 16, in essence, mirroring one-half of the anti-cross thread 16. The base of this curved lead thread flank 17b shares its root with the anti-cross thread 16. As such, its root appears to be a continuation of the anti-cross thread's root, and half of the lead thread profile appears to be an anti-cross thread profile. As this flank approaches the end of the helix, the curved surface becomes increasingly narrower, finally disappearing at the end of the helix as the lead thread reaches zero height. The other lead thread flank 17a tends to be essentially flat and angled to the axis of the fastener, at an angle and flatness as well as convexity and concavity that typically vary significantly and freely in every section, depending on the location on the helix as well as manufacturing technique. Typically, the angle, convexity or concavity of this flank is not controlled during manufacturing and varies significantly in angle, growth rate, and contour throughout the lead thread's length. As the lead thread traverses around the body, this “flat” flank 17a tends to narrow and shrink in height. This “narrowing” is the result of the root of the flat-angled flank moving closer and closer to the root between the lead thread 17 and the anti-cross thread 16, as the lead thread 17 traverses around the shank. Ultimately, this flat-angled flank 17a narrows to zero as the height and breadth of the lead thread 17 decreases toward the end of the helix. Therefore, as this pointed peak of the lead thread is typically formed by unrestricted free-flowing metal, it tends to vary significantly in shape throughout the length of the lead thread, as well as from fastener to fastener.
Third, the flanks of the lead thread 17 tend to form a point rather than a flat or curved crest at the intersection of the two flanks.
FIGS. 8A-8C illustrate cross-sectional side views of a typical process for manufacturing a fastener from an unthreaded fastener blank by rolling it between opposite thread rolling dies. In FIG. 8A, thread rolling dies 40 are separated to demonstrate placement of the fastener blank 41 prior to rolling. The shank 12 of the fastener blank 41 usually has a constant diameter to allow formation of standard threads, and the lead end 14 of the unthreaded fastener blank usually has a chamfer 9 to allow formation of a lead thread. FIG. 8B shows the thread rolling dies 40 in a rolling position so that the threads are being formed on the fastener blank to form male fastener member 10. In typical manufacturing practices, used for both standard and anti-cross thread fasteners, the threads may be formed by rolling an essentially cylindrical, unthreaded fastener blank 41 through thread rolling dies 40. The dies 40 impart the thread profiles onto the unthreaded blank 41 by displacing metal into grooves formed in the thread rolling dies. As shown in FIG. 8C, a fastening thread 15 may be formed on the constant diameter shank portion of the unthreaded fastener blank by allowing the metal to flow into grooves of the dies until the grooves are completely filled with metal from the blank. The fastening threads 15 formed on the shank of the fastener tend to have completely uniform profiles because the metal completely fills the grooves in the dies. The lead thread 17, however, may be formed by incompletely filling grooves of the rolling dies 40 with metal from the chamfered end of the fastener blank. In the lead thread section of the fastener, the thread helix is formed by only partially filling the grooves in the rolling dies. Partial filling occurs because the fastener blank has a chamfer at its end, as shown in FIG. 8A.
FIG. 8C illustrates a close-up view of the lead thread section shown in FIG. 8B. Because the unthreaded fastener blank has a chamfer at its end, there is not enough metal present in the lead thread area to completely fill the grooves in the thread rolling dies 40. The lead thread 17 for both standard fasteners and anti-cross thread fasteners is formed by partially filling the die grooves in the lead threaded section. Partial filling allows the metal to freely flow into a variety of lead thread profiles as described above. Thus, lead thread profiles are inherently inconsistent due to blank, die, and process variation. In most known lead threads, variation is inherent because of the thread rolling dies allow unrestricted free-flowing metal to take a variety of lead thread profiles.
Some lead thread profiles are particularly susceptible to false threading. In some cases, the lead thread profile varies too much over the length of the lead thread, so as to cause false threading. For example, the lead thread may increase in height too quickly, i.e., it grows from the shank to the full height of a standard thread in less than 180° around the shank. For another example, the angle of the front flat-angled flank of the lead thread may be too steep to allow proper mating with female threads. As a further example, the lead thread profile may be severely pointed, which can lead to possible false threading and/or unintended and undesirable contact with internal female threads during initial threading. For anti-cross thread fasteners, some lead thread profiles may cause the anti-cross thread fastener to false thread in internal female threads before the anti-cross thread has an opportunity to align the fastener members for proper threading. Some lead thread profiles cause the internal and external threads to contact each other in an undesirable location on the respective helices, before the anti-cross thread acts upon the internal thread to align the fastener members, thus hindering and/or preventing the anti-cross thread from camming over the internal lead thread of the female fastener member. This may be especially true when initial angular misalignment of the two fastener members is high. Additionally, many lead thread profiles on an external male fastener member may engage a groove in the internal female lead so as to cause a false threading condition, as described above.
Some lead threads used on anti-cross thread fasteners have a steep helix angle, such that the lead thread grows from zero height to the height of the anti-cross thread in just 270° around the shank. The peaks of short lead threads such as these may engage the groove in the lead threads of internal female fasteners and/or the smaller grooves in the internal full threads, which leads to false threading.
In still other lead threads used on anti-cross thread fasteners, the width of the lead thread profile is maintained constantly to be similar to the width of the standard thread, as the height of the lead thread increases over the length of the lead thread. These lead threads tend to form a profile, which is different in every section of the lead thread. They tend to have a very flat curve near the beginning of the lead thread, and a progressively smaller, more pointed profile as the lead thread helix progressively blends into the profile of the anti-cross thread. These lead threads tend to lodge in the groove at the tip of the internal female lead thread as the lead thread grows in height, which leads to false threading.
What is needed, therefore, is an external lead thread of a male fastener member that tends to prevent false threading of the lead thread into a groove in the peak of the internal thread of a female fastener member. The lead thread should not be subject to manufacturing variations causing undesirable localized lead thread heights and profiles, and large helix angles. For anti-cross thread fasteners, a lead thread is needed that facilitates, rather than hinders, the performance of the anti-cross thread.